Untangling the Many Ways Ocean Acidification Harms Marine Life

By Craig Welch

Photo: David Liittschwager, National Geographic Creative

Inside a laboratory with massive saltwater tanks, sharks attack a whiff of squid odor pulsing through the water, much as they might chase dinner in the ocean. That scent, after all, represents a potential meal, and sharks are basically smelling machines with teeth.

But when scientists in this recent experiment added carbon dioxide to the water, these sharks, called smooth dogfish, suddenly seemed to lose interest in hunting food.

Marine biologist Danielle Dixson wasn’t surprised. Over the last nine years, Dixson has become a leader among those tracking the many ways that souring seas with CO2 — called ocean acidification — can harm marine life. And what scientists are learning suggests acidification may prove more troubling than previously thought.

From the ability of sharks to sniff out food, to how well clownfish see and hear, to whether snails can escape being eaten by sea stars, acidification poses grave risks to ocean life beyond shellfish and coral. And these changes threaten to reverberate through the marine food web in ways we can’t predict.

“It’s going to affect pretty much everything in the ocean in one way or another,” says Dixson, an assistant professor at the University of Delaware.

As we pump ever more CO2 into the atmosphere, nearly a third of it gets absorbed by the sea, where it alters the chemistry of marine water. All that CO2 is lowering the oceans’ pH, making waters more acidic faster than at any time in 55 million years. But CO2 changes water in other ways, too, robbing it of the calcium carbonate that shell-building creatures rely on to grow.

Scientists long have known that this phenomenon poses grave problems for oysters, corals, mussels, and small sea butterflies known as pteropods, a staple in the diet of many salmon and whales. Now Dixson and others are showing that acidification also can upend the behavior of fish and other creatures, with potentially far-reaching implications.

“One of the things we’re rapidly learning is that we should expect the unexpected,” says Tessa Hill, an acidification expert at the University of California, Davis’ Bodega Marine Laboratory. “Acidification has the potential to really, really change the structure of the ocean—who lives there, where, and how many there are. And that will fundamentally change our relationship with the ocean.”

Castello Aragonese, a volcanic island off Naples, Italy, healthy seafloor looks like this: a lumpy quilt of red sponges, white barnacles, lilac coralline algae, sea urchins, and (near the center of the photograph) one well-camouflaged fish. Photo: David Liittschwager, National Geographic Creative.A few hundred yards from the preceding scene, CO₂ bubbling from seafloor vents acidifies the water to levels that might one day prevail all over the oceans. Dull mats of algae replace the colorful diversity—”fair warning,” says biologist Jason Hall-Spencer. Photo: David Liittschwager, National Geographic Creative.

Scrambling Fish Brains

Dixson was there at the beginning. As a graduate student in Australia almost a decade ago, she studied baby damselfish on Papua New Guinea’s tiny Kimbe Island. Dixson wanted to figure out how the creatures used chemical olfactory clues to key in on specific reefs. Just a few years after the debut of the film Finding Nemo, Dixson was learning how clownfish used smell to find their homes.

She teamed up with a colleague studying how reef fish responded to acidification so the two could explore the impact on clownfish senses. Their findings would refocus climate change research in the ocean.

Exposure to high CO2 messed up the fishes’ ability to distinguish between smells, as well as affecting their hearing and sight. Juveniles did not avoid predator sounds, were less attracted to the smell of food — and more likely to swim smack into predators wanting to eat them even after seeing them, leading to higher death rates. Later experiments showed similar responses by important coral trout and some types of wrasse, and opened a new field of research examining how acidification alters marine life behavior by affecting receptors in the brain.

In just the last five years, scientists have learned that acidification can make three-spined sticklebacks more shy and less likely to escape predators. California rockfish spend more time hiding in the dark. Pygmy squid, on the other hand, tend to move more often. Small-spotted catfish sharks changed how often they turned left or right. Snapping shrimp, some of the oceans’ noisiest creatures, snapped more quietly and less often, reducing overall sound in some coastal regions. Some species of fish in the same family responded to acidification in completely different ways from each other.

Most research has been conducted on tropical fish. But there are troubling signs for other species, too. When baby red king crab, the lucrative Bering Sea shellfish made famous by the television show The Deadliest Catch, were exposed to high CO2 in the lab, they grew slower and eventually all died. Baby Atlantic cod, exposed to levels of CO2 expected by century’s end, also died more often than they did in normal water.

It’s far too soon to know just what it all these changes mean—for the future of individual species, for the ocean food web, and for us. There are still too many variables.

Coping with Change

CO2 levels in the ocean fluctuate wildly with the tides and the seasons, especially in estuaries and along many shorelines. There is growing evidence that some species in those environments, such as sea urchins, may be able to adapt to acidification, even at its current rapid clip. And yet other evidence suggests that some behavioral maladies in marine creatures won’t subside even after multiple generations have been exposed.

Complicating matters, ocean acidification isn’t happening in a vacuum. Rising temperatures, in some cases, appears to slow the kind of havoc brought by acidification. But then, warm waters will bring troubles of their own. And in some cases, all these multiple stresses on marine creatures can make things far worse.

One of the most important fish along North America’s East Coast is a tiny baitfish called the Atlantic silverside, which is eaten by many birds, crab, turtles, and larger fish. When waters in estuaries such as Chesapeake Bay get too warm or polluted, they lose oxygen, which can kill silversides. Unfortunately new research shows that acidification can make low-oxygen waters even more deadly.

“What we saw was that when the water chemistry changes, even if there’s a little more oxygen in the water, they’re more likely to die,” says researcher Seth Miller, with the Smithsonian Environmental Research Center in Maryland.

Some degree of change is inevitable. Even if we halt fossil-fuel burning today, there will be a lag time before sea chemistry stops changing. The ocean will undergo at least a bit of reshuffling. But how severe it gets is up to us.

“There are times when this feels very overwhelming to me, but I’m not interested in documenting the ocean’s decline,” says Hill. “I’m interested in documenting the way to go. We can control the trajectory of this. It’s completely under our control. We just have to start making smarter decisions. And we have to start making them now.”